Many commercial polymers exist in the marketplace today. These polymers have a variety of applications and are exposed to a variety of environments. The value of many of these commercial polymers can be increased by enhancing the surface or interface properties of the polymers in a cost-effective manner. Examples of these properties include but are not limited to: adhesion, permeability, anti-soiling, and antimicrobial properties. Polyesters (in particular, polyethylene terephthalate (PET)) are of particular interest because of their widespread use in textiles and packaging. The processing of these materials is highly cost-sensitive.
Some polymers contain reactive or potentially reactive carboxylic acid surface sites that are useful for a number of applications. Such applications include grafting, wetting, dyeing, and adsorption. However, common commercial polyesters (such as PET) have very few acid sites, making it difficult to modify the surface of the polyester via chemical reaction. In an effort to improve surface modification, methods have been developed to add carboxylic acid sites to the polymer surface. However, providing acid sites to these polymers by modifying their bulk chemistries (for example, by incorporating additives) increases cost and process complexity, and may adversely affect other physical or chemical properties of the polymers. Alternatively, providing carboxylic acid sites by coating a polymer film with a layer of a polymer having the desired functionality adds cost and weight, and may involve the use of environmentally unfriendly or expensive chemicals and processes.
A need exists for a process wherein the concentration of acid groups on the surface of polymers is increased without adversely affecting the bulk properties or the surface topography of the polymer. The present invention solves this problem by providing a process for surface modification that utilizes deep ultraviolet (UV) irradiation. There are few reported studies of the effect of deep UV (that is, wavelengths below 250 nm) on PET surface chemistries. No effect was found by X-ray photoelectron spectroscopy (XPS) for excimer laser irradiation in air at 248 nm of biaxially oriented PET film at 12 mJ/cm2 (below the ablation threshold) (Dunn, Ouderkirk, Macromolecules, Vol. 23 (1990) p. 770). Upon irradiation at 193 nm in a nitrogen atmosphere, a loss of oxygen-containing species is evident by XPS even at 10 mJ/cm2 (Chtaib, et al., J. Vac. Sci. Technol., Vol. A 7 (1989) p. 3233). This study showed that C—O bonds were affected more than C═O bonds. Even though the fluence in both excimer laser studies was below the ablation threshold, thermal effects were not absent, since the polymer was amorphized in both cases. Irradiation with 185 nm Hg vapor light in vacuum led to reduced surface oxygen as seen by XPS, but increased uptake of derivatizing reagents for carboxyl functionality. In contrast, repeating the experiment with a filter that excluded 185 nm emission and passed only the 254 nm component resulted in little or no effect (Lazare, Srinivasan, J. Phys. Chem., Vol. 90 (1986) p. 2124),consistent with the 248 nm observations. Irradiation in nitrogen at 222 nm (KrCl excimer lamp) resulted in a modest increase in surface oxygen species as seen by XPS, but only after 500 J/cm2 (Praschak, et al., Appl. Phys., Vol. A 66 (1998) p.69). At this fluence level, the question of a possible role for residual oxygen in the treatment cell atmosphere is hard to avoid.
Grafting implies attachment of a species to the polymer surface by covalent chemical bonds. Among the processes that can be used to obtain a desired surface chemistry, grafting is particularly appealing in that added species are strongly attached, the amount of added material is minimized, and the prospects for adversely impacting the original properties of the substrate are minimized. Since grafting is a chemical reaction, the substrate to be coated and the grafting reagent must provide reactive sites, or reactive sites must be created during the process. Wet chemical methods are well-known to those skilled in the art, but require contacting the substrate surface with fluids containing the required chemical species. Application, removal, and disposal of successive fluids contributes complexity and thus cost to a grafting process. Relief from these burdens may be sought by using an appropriate wavelength of light to supply the energy needed to drive the chemistry. As described in a recent review [Rånby, Yang and Tretinnikov; Nucl. Instrum. Meth. Phys. Res. B 151 (1999) 302-305], a mixture of photoinitiator and grafting reagent is applied to a polymeric substrate surface which is then exposed to sufficient UV light of the desired wavelength. This process requires the costs of an added ingredient (the photoinitiator) and an added step (application prior to UV irradiation). There is a need for a simpler, cheaper process.
An object of the present invention is to provide a process for modifying the surface of a polymeric material that does not adversely affect the bulk properties of the polymer.
Another object of the present invention is to provide a process that allows full surface coverage with a thickness of only about one molecular layer.
Another object of the present invention is to provide a process that permits penetration of a reagent into open structures in the polymeric material.
Another object of the present invention is to provide a process that is simple and cost-effective.
Another object of the present invention is to provide an efficient vapor-phase grafting process that does not use photoinitiators
Another object of the present invention is to provide a one-step UV grafting process.